Crazy Cactus

[Wasel Chemij]

I was given a cactus last year and it's beginning to misbeahave
insofar that it's putting out long roots from this years new growth.
I'm not sure of the name of the cactus but it's the type that has
'leaves' with a triangular section and spines only on the edges.
The leaves are about 4-5 cm wide and 10-15cm long.  Each years growth
ends in a point and the following years growth starts at that point.
My cactus has three sections and no side leaves.  There are now 4 roots 
growing from the smooth, un-spined faces of this years growth.  The 
oldest root is 20-25cm long and the youngest is 4cm long.  There is no
evidence of this happening on any of the other sections.  The cactus
is on a window ledge with good light, is growing in well drained
soil and it watered whenever the surface soil becomes dry.
Why is it putting out roots from the top?  What if anything should 
be done about it?

				Vas